The invention relates to a method for determining the erosion of contacts of an electromagnetic switching device. In addition, the invention relates to an electromagnetic switching device with a device for determining the erosion of its contacts.
During the switch-on and switch-off operations of an electromagnetic switching device, arcs occur between the closing or opening contacts. These arcs cause erosion of the contacts over the course of time. It is therefore important for the operational reliability of such a switching device to identify the degree of this erosion in order to be able to draw conclusions on the residual life of the switching device and avoid operational faults by replacing the contacts in good time.
EP 0 694 937 B1 has disclosed a method for determining the erosion and therefore the residual life of contacts in switching devices, in which method the so-called contact resilience is determined as a measure for the contact erosion. This contact resilience is the distance which is covered by the magnet armature as the actuator of the switching movement between the beginning of the switch-off operation, i.e. the time at which the magnet armature, which is resting in the end position on a magnet yoke, releases itself therefrom and the time at which the contacts lift off from one another. The time at which the magnet armature lifts off from the magnet yoke is measured by an auxiliary circuit, in which the magnet armature and the magnet yoke form a switch, which is closed if the magnet armature and the magnet yoke are in contact with one another.
As an alternative to this, it is known, for example, from EP 0 878 015 B1, to determine the time at which the magnet armature separates from the magnet yoke of the magnet drive by measuring the voltage at the magnet coil of the magnet yoke.
In both methods, a further auxiliary circuit is required for detecting the time at which the contacts lift off from one another, for example a complex auxiliary circuit which is DC-decoupled from the main circuit with the aid of optocouplers and which detects the occurrence of an arc voltage, which is produced by the arc forming when the contacts lift off from one another.
As an alternative to the methods known from EP 0 694 937 B1 and EP 0 878 015 B1, in which the switch-off operation is used to determine the erosion or the residual life, WO 2004/057634 A1 has disclosed a method and an apparatus for determining the residual life of a switching device, in which method the change in the contact resilience is measured during the switch-on operation, i.e. when the switching contacts are closed by the magnet drive. With this known apparatus, a position encoder is arranged on the magnet armature, which position encoder contains markings, for example in the form of measuring contacts, in at least three positions, with which markings the time profile of the magnet armature movement can be detected. The determination of the position of the magnet armature when the contacts close is determined by computation from the movement sequence of the magnet armature which is detected with the aid of these position markers. For this purpose, a simple algorithm is used as a result of the low number of position markers assuming that the armature acceleration is constant between a time prior to the closing of the contacts and a time which is between the closing time of the contacts and the time at which the magnet armature is positioned onto the magnet yoke. In practice, however, it has been established that, with such an approach, the time at which the contacts close can only be determined with a low amount of accuracy.